This invention relates to a rotational speed difference sensitive type joint assembly disposed between two members which are capable of rotation relative to each other, and for generating a resistance of the size corresponding to the difference in speed between the two members.
The conventional joints of this type include a housing and a rotor which are capable of rotation relative to each other. The rotor is provided with a plurality of cylinder bores. Each cylinder bore receives therein a plunger such that the plunger is capable of reciprocal movement within the bore. Each plunger is urged against a cam face formed on the housing by a spring. Therefore, when the housing and the rotor are rotated relative to each other, each plunger is reciprocally moved. When the plunger moves in one direction, working oil in the cylinder bores flows into a working oil receiving chamber, and when the plunger moves in the other direction, the working oil in the working oil receiving chamber flows into the cylinder bores (see Japanese Patent Unexamined (Kokai) Publication No. 159423/90, Japanese Patent Examined (Kokoku) Publication No. 103902/95, USP495473, etc.).
The working oil within the cylinder bores passes through an orifice when it flows into the working oil receiving chamber. The working oil is subjected to flowing resistance when it passes through the orifice. The working oil within the cylinder bores are pressurized by this flowing working oil within the cylinder bores are pressurized by this flowing resistance. The pressurized working oil urges each plunger against the cam face to generate frictional resistance between each plunger and the cam face. The housing and the rotor are limited in their relative rotation by the flowing resistance which occurs when the working oil passes through the orifice and by the frictional resistance which is generated between each plunger and the cam face. In other words, the torque corresponding to the flowing resistance and the frictional resistance is transmitted from the housing (or rotor) which is rotating at a high speed, to the rotor (or housing) which is rotating at a low speed.
When the housing and the rotor are rotating relative to each other, the working oil is heated by heat generated when the working oil passes through the orifice or by heat caused by the friction between the plunger and the cam face. On the other hand, when the housing and the rotor are stopped, the working oil is cooled by natural radiation. Therefore, the working oil repeats expansion and contraction. For this reason, if the volume of the working oil receiving chamber is constant, the pressure of the working oil is greatly fluctuated.
In view of the above, in a rotational speed difference sensitive type joint assembly disclosed in the Japanese Patent Unexamined (Kokai) Publication No. 159423/90, a piston biased by a spring towards an inner side of the working oil receiving chamber is slidably disposed within the working oil receiving chamber, the expansion and contraction of the working oil are absorbed by moving the piston in accordance with the expansion and the contraction of the working oil, and the pressure within the working oil receiving chamber is maintained generally constant.
However, in the above-mentioned conventional rotational speed difference sensitive type joint assembly having a piston in the working oil receiving chamber, various kinds of parts, such as a piston, a spring and a seal member for sealing a space between the piston and the cylinder bore are required. Therefore, the number of parts is increased and the manufacturing cost is increased to that extent. Further, since the piston repeats its reciprocal movement in accordance with the expansion and contraction of the working oil, the seal member is gradually worn and the working oil within the working oil receiving chamber is liable to leak outside.